Desalination system

ABSTRACT

The invention provides methods and an apparatus for more efficiently and economically producing purified water from sea water or some other salty or brackish water source. The efficiency is derived from the co-location with a power plant or other thermal generating source that will heat the feed water. Reverse osmosis membrane filtration systems work optimally when the feed water is at certain higher temperature, where that temperature is typically higher than the feed water at ambient temperatures. By using the heated sea water as the byproduct of the power plant electricity generating process and if necessary mixing it with ambient temperature sea water, if needed to lower the water temperature, and using this feed water with a higher temperature than ambient water temperature, the efficiency of the reverse osmosis system can be increased.

This application is based on U.S. provisional application Ser. No.60/343,231, filed Dec. 31, 2001.

BACKGROUND

The present invention relates generally to desalination of seawater and,more particularly, to methods and an apparatus for desalination ofseawater using reverse osmosis membranes.

The desire to make drinkable, potable water out of seawater has existedfor a long time. Several approaches can be taken to remove the salt andother chemicals. Water distillation is one way to approach the goal, butmay not be commercially feasible. In this approach, water is heated toseparate the solids from the liquid and therefore remove the saltsolids. Another approach is electrodialysis in which the ions formingthe salt are pulled by electric forces from the saline water throughmembranes and thereafter concentrated in separate compartments. Thisapproach is also very expensive. A third approach to desalination isthrough reverse osmosis. This method uses pressure to force salty feedwater against membranes which allows the relatively salt free water topass through, but not much of the salts or other minerals. But due tothe high production and capital costs, desalination systems are notwidely used for making large scale supplies of public drinking water.

Efforts have been made to increase the efficiency of reverse osmosissystems in general and specifically, with respect to a desalinationsystem, to lower operating and fixed costs. Some efforts have beendirected at the improving the efficiency of the filtration systems,other efforts have been directed at the design and application of filtermembranes, multi stage filtering and nano-filtration methods. Otherefforts have also been directed at improving the efficiency of otheraspects of a reverse osmosis system. For example, some efforts have beendirected at the membrane filtration system replacement method bymonitoring the silt density and at the application of particular feedwater pressures.

Each of these efforts may increase the efficiency of the desalinationsystem, but these efforts may not sufficiently reduce the cost of thesystem for use for public ware supply. What is needed is a desalinationsystem that processes seawater into potable water more cost effectivelyfor use for public water supply.

SUMMARY

The present invention provides a purification system for desalinatingwater having two water supply sources: an ambient feed water source forinputting ambient feed water; and a high temperature feed water sourcefor inputting high temperature feed water, where the high temperature isa temperature higher than the ambient temperature. A blender mixestogether the ambient and high temperature feed water to achieve ablended feed water temperature in a desirable range. The blended feedwater is then desalinated in a reverse osmosis process.

In one aspect, the second source of water is derived from a power plantthat uses seawater as a coolant and outputs heated seawater through ahigh temperature feed water line. The water supply for seawater coolantfor the power plant may be the same water supply used for the ambientwater line.

In another aspect, the first source of water for ambient temperaturewater is drawn by an ambient variable speed pump, which is controlled bya controller and is output through a feed water line to a blendingprocess. The heated water source of water is drawn by a high temperaturevariable speed pump, which is controlled by a controller, and is outputthrough a feed water line to a blending process.

In another aspect, using temperature sensors located in the ambient feedwater line, the high temperature feed water line, and the output of theblending process the controller mixes the two streams of water toachieve a desired temperature of blended water.

In another aspect, the controller may adjust the mixture based on thedesirability of other factors, such as water salinity. The blended wateris then pretreated which separates the water into solids, unusable waterand usable water. Unusable water is delivered to a discharge area whichmaybe the same discharge area used by the power plant. The pretreatmentsolid waste is delivered to a landfill. The usable water is thendelivered to the desalination process.

In yet another aspect, in the desalination process, a pump, controlledby a controller, delivers the water to a first stage reverse osmosisfilter. The controller may adjust the pump by using input fromtemperature or water pressure sensors located in the water line inbetween the pump and the filter. The water is then filtered intopotentially usable and unusable water. Unusable water is delivered to adischarge area and may pass through an energy recovery pump. Potentiallyusable is then separated into usable water and water that requiresfurther filtration. Some of the water is passed through a pump and thendelivered to a second filter process. The controller may adjust the pumpby using input from temperature or water pressure sensors located in thewater line in between the pump and the filter. The water is thenfiltered into potentially usable and unusable water. Unusable water isdelivered to a discharge area. Usable water is then mixed with usablewater from the first filtration and delivered into a storage tank. Thiswater may be then treated for lime stabilization and then chlorinationand then stored for use.

Other embodiments of the invention may use different desalinationprocesses. Approaches may utilize variations on the number and type ofreverse osmosis filters. Other approaches may apply a desalinationtechnique other than reverse osmosis.

These and other features and advantages of the invention will be moreclearly understood from the following detailed description and drawingof a preferred embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic illustration of a desalination systemaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to the FIGURE, there being shown a desalination system,generally designated by reference numeral 110, according to a preferredembodiment of the present invention. Desalination processes forproducing purified water from seawater or some other salty or brackishwater source, generally work more efficiently at particular ranges oftemperature, depending on the nature, geometry, pressure and otherparameters of the given desalination process. The embodiment illustratedin the FIGURE uses first and second pass reverse osmosis filterprocesses 70 and 90 for desalination.

Separation of liquids from particulate solids, and colloidal and ionsize particles can be achieved by a number of technologies usingmicrofiltration, ultrafiltration, nanofiltration and reverse osmosismembranes. These technologies have been commercially available andproven on a number of full-scale installations worldwide over the last20 years. However, the wide application of the membrane separationtechnologies have been limited by the high power costs related topressurizing the treated liquid against the membranes. This isespecially true for seawater desalination technologies using reverseosmosis membranes, where the operating pressures required to separatemonovalent and divalent ions from the intake water are very high (aboutthree hundred (300) p.s.i. to about twelve hundred (1,200) p.s.i.).

One of the key factors influencing the pressure required to achieve agiven level of product water quality is the temperature of the water fedto the membrane elements. The effect of temperature on reverse osmosisand nanofiltration membranes is driven by the phenomenon of increasedsystem enthalpy. As the temperature decreases, the membrane elementmaterial becomes more rigid and water flux through the membranesdecreases. Increase in feed temperature relaxes the bonds within themembrane matrix and water and salt molecules move more rapidly. As aresult of the looser matrix structure, water passes through the membranepores at lower applied pressure than the pressure needed at lowtemperatures.

For a temperature range between about five (5) and about forty (40)degrees Celsius, the pressure required to achieve the same membranewater productivity (flux) decreases proportionally to the increase intemperature. Typically, about eighty percent (80%) higher pressure, andtherefore correspondingly higher power use, is needed to produce thesame water quality and quantity when the feed water temperature is aboutfive (5) degrees Celsius compared to that when the feed watertemperature is about twenty (25) degrees Celsius.

The beneficial effect of feed water temperature is observed fortemperatures of up to about forty (40) degrees Celsius. Highertemperatures, e.g., temperatures above forty (40) degrees Celsius,result in compaction of the membrane material, which ultimately causesreduction of the membrane's useful life and significant deterioration ofproduct water quality.

It should be noted that an increase in temperature results in anincreased passage of both water and ion molecules through the membranematrix. Because the membrane material is specifically designed to rejectsalts, rather than water, the magnitude of increase in product watersalinity in the final product with increase in temperature, issignificantly lower than the increase of membrane flux or decrease ofpressure. As the temperature of the feed water rises, the feed pressuredecreases and the product water salinity increases slightly.

Product water salinity increases at significantly lower rate than therate of decrease of pressure needed to produce this water. Therefore, tothe point the actual product water salinity reaches the target productwater salinity concentration, the overall effect of feed watertemperature increase on water production economics is positive. Thisoperational point of the system is called “optimum temperature point.”At this optimal temperature, target product water salinity is achievedat minimum pressure and/or maximum flux. If the feed water temperatureexceeds the optimum point, the product water salinity deteriorates to alevel where the system capacity has to be reduced below its target valueto produce the same water quality.

It should be noted that the optimum temperature point for a givenmembrane system and feed water source depends on many factors including:the actual feed water salinity; the target product water salinity; thecontent of substances with high scaling potential in the feed water, pH,and other water quality parameters. See, for example “Manual of MembraneProcesses for Drinking Water Treatment,” by Mark A. Thompson, et al.,October 1996, Malcolm Pirnie. Most important of these control parametersare the intake and the product water salinity concentrations. Typically,for a given membrane system the product water salinity is a constanttarget set by the type of use of the water, e.g., potable, industrial,or other use. Therefore, generally, the main control parameter is theintake water salinity. If intake water salinity increases, typically theoptimum temperature point value decreases. Therefore, by controlling theintake water temperature the system power demand can be minimized(continuously operated at the optimum temperature point) under changingintake water salinity and/or other water quality parameters.

Currently, typical sources of water for seawater desalinationinstallations are brackish groundwater pumped using wells and/orseawater collected directly from the ocean via an open intake structure.Seawater and groundwater have relatively low temperatures, whichfluctuate daily and seasonally. Currently, desalination waterinstallations are designed for the lowest occurring daily averagetemperatures to secure consistent product water quantity and quality atany given time. To compensate for the negative effect of low temperaturefeed water on the system design, additional number of membranes and/orhigher feed pressures (larger pumps and higher rating piping) must beapplied. These additional membranes and the installations housing themincrease the overall membrane system costs. In addition, at lowertemperatures the desalination system has to be operated at highpressures to produce the same water quantity, which results in highpower costs and ultimately shorter membrane useful life.

The desalination system of the illustrated embodiment reducesdesalination facility water production costs by automatically adjustingthe membrane feed water temperature to its optimum temperature point toaccommodate changing feed intake water temperature and quality at thesame time maintaining consistent product water quality at a targetlevel. The system includes the following elements: Source and intake ofseawater of ambient temperature; Source and intake of seawater ofelevated temperature; Feed pumps and pipelines conveying seawater fromthe high- and low-temperature water sources to the desalinationfacility; Blending device for the high-temperature and low-temperaturefeed water stream; Variable frequency drives (“VFD”) installed on thelow-temperature feed water pumps; Water conductivity/salinity metersinstalled on the high-temperature and/or low-temperature feed lines;Water temperature sensors and signal transmitters installed on thehigh-temperature, low temperature and blended feed water lines; Membranesystem differential pressure sensors, transmitters and controls; andMonitoring and control instrumentation allowing adjustment of blendedfeed water temperature as a function of the membrane system differentialpressure readings/signal and product water salinity.

Typical sources of intake water of elevated temperature are power plantoutfalls. The source of intake water of ambient temperature could be theintake wet well of the power plant feed pumps or a separate seawater,groundwater or freshwater intake. The prime source of feed water is theelevated temperature source. The two key control parameters of thetemperature adjustment system are product water salinity and membranedifferential pressure.

Thus, to enhance the efficiency of the desalination process, it isdesirable to maintain the temperature of the water at the membranewithin a predetermined range of temperature.

In the illustrated embodiment, two supplies of seawater are provided.The first supply 600, referred to as the ambient water, is seawatertaken from the sea, bay or other source and is at a relatively lowtemperature. The temperature of the ambient water may be different fromthe original source, because of heating or cooling caused by, forexample, handling, pumping, exposure to solar heating or heating orcooling by the atmosphere. The second supply 400, referred to as thehigh temperature water is seawater that has been delivered through thecooling system of a power plant 500. The high temperature water shouldnormally be substantially hotter than the ambient water.

The high temperature seawater is mixed as necessary with the ambienttemperature seawater to achieve a temperature of the seawater at themembranes in the range of desired temperatures to thus enhance theefficiency of the reverse osmosis process. Additional efficiency resultsfrom the location of the desalination process near a power plant orother thermal generating source, so that waste heat may be used to heatthe high temperature water. This reduces or eliminates the need toprovide supplemental heating for the seawater in the desalinationprocess. Also, the efficiency of the reverse osmosis system can beenhanced by monitoring conditions such as pressure, temperature andsalinity and adjusting the temperature, salinity and pressure of thefeed water in response to those conditions. In addition, the use ofenergy recovery systems, such as heat recovery turbines, may alsoincrease efficiency of the desalination process.

As shown in the FIGURE, water is drawn from sea water supply 18, such asan intake canal off an ocean, salt water lake, bay, or other watersource, into a first intake 33 and a second intake process 35 by pumps(not shown) through a first and second sea water input line 13 and 15.Sea water input lines 13 and 15 include a course or rough filter toremove large sediment, debris and fish from the seawater. The water isthen delivered from the intake processes 33, 35 to the power plant 500through power plant input lines 45, 46, 47, and 48. Water is alsodelivered from the intake process 35 to a first variable speed pump 32,and then delivered through an ambient feed water line 42. The watersupplied through ambient feed water line 42 may also be referred to asthe ambient water. The variable speed pump 32 is electronicallycontrolled by input from controller 10. The variable speed pump 32 isconnected to the controller 10 through appropriate and necessaryelectrical connections. Connections for the first variable speed pump 32and other components to the controller 10 are not shown in the FIGURE.Controller 10 may provide for both manual and automatic control of thesystem. Although this embodiment provides for a plurality of intakesystems and intake lines, a plurality is not required.

A power plant 500, as part of its normal operation, will utilize a localwater source to input a stream of water to absorb heat that results fromthe power generation process and output the water at a relatively highertemperature than when it was first drawn from the water supply. In theFIGURE, water will be output through power plant output lines 20, 22, 24and 26. One of the sources of increased efficiency in the presentinvention is the use of heated supply water which is a byproduct of thepower plant. Other embodiments may utilize other heat generating sourcesthat generate heated water as a by product of the system; thereforesupplying heated water without significantly increasing the cost of thedesalination system.

Heated water leaves power plant 500 through power plant output lines 20,22, 24, 26 and is directed to discharge 14. Diverters 11, 12 located inpower plant output lines 24 and 26 direct water through lines 17 and 19,respectively, to a high temperature water pump station 400 and thesecond variable speed pump 36 contained within. Controller 10 monitorsthe water temperature in lines 17 and 19 through temperature sensors 21and 23, located respectively with those lines. Controller 10 controlsthe flow of water from line 17 and 19 into high temperature pump station400 by adjusting gates 25 and 27, which are located in betweentemperature sensors 21 and 23 and high temperature water pump station400. The controller 10 may select water from a combination of lines 24and 26 dependant on the temperature of the water in lines 17 and 19 andthe desired water temperature desired by blending process 300, discussedbelow. The resulting water is output from the high temperature waterpump station 400 by variable speed pump 36 through a high temperaturefeed water line 44.

In the blending process, the ambient and high temperature water aremixed. Ambient temperature feed water, flowing through line 42, and hightemperature water feed water, where the high temperature water is waterthat has a relatively higher temperature than the ambient water, flowingthrough line 44 merge in blending process 300. Controller 10, usinginput from temperature sensors 34, 38, and 40, and possibly input fromsalinity sensors 37 and 39, which are located in lines 42 and 44,respectively, just prior to blending process 300, adjusts variable speedpump 36 and 32 and blender 41 to attain the preferred temperature rangeof feed water. The resulting water is output through a blended feedwater line, line 52. The determination of the preferred temperatures isknown to those skilled in the art. In another embodiment, it might bedesirable to adjust the blending process to reach a preferred watersalinity level, or, possibly, a combination of both factors.

In a preferred embodiment of the invention, water goes through aninitial pretreatment filtering process. After blending, the feed waterenters the pretreatment process 200 through line 52 wherein the feedwater undergoes an initial filtering process. In this step, the incomingwater is separated into solids, usable water, and undesirable water.Unusable water leaves pretreatment process 200 proceeds to discharge 14through a pretreatment unusable water line, line 56. Solids that resultfrom pretreatment process 200 are delivered to landfill 16 through apretreatment solid waste line, line 58. The remaining, usable, waterleaves through a filtered feed water line, line 54.

Membrane filters may require pre-treatment filtering directed atadditional water characteristics, such as acidity.

A preferred embodiment of the invention employs a two (2) stage, or two(2) pass, reverse osmosis filter process as the desalination method.Feed water enters the desalination process 100 though line 54 where itis directed to a first membrane filter, a first feed pump 80, thenproceeds into a first membrane filter, first pass filter process 70,through a first purification pump line, line 85. First pass filter 70has a membrane 102 which separates relatively salty water 101 fromrelatively less salty water 103. Controller 10 adjusts the pressure ofthe water delivered to membrane filter by input from pressure sensor 86,which is located in line 85, and controlling pump 80. Controller 10 mayalso utilize temperature, as determined from temperature sensor 61located in feed line 85, and salinity, as determined from salinitysensor 63 located in feed line 85, to adjust the pressure. Thecontroller 10 may also adjust the water pressure delivered by the pump80 by measuring the differential pressure in the first stage filter 70.

Saltier water, unusable for purification, leaves first pass filterprocess 70 through a first membrane unusable line, line 78; and theremaining water leaves first pass filter process 70 through a firstmembrane output line, line 75, where it is delivered to separator 76.Separator 76 separates some of the water, preferably less than abouthalf and more preferably about 5% to about 15%, and delivers that waterthrough a first membrane potentially usable line, line 74, into a secondmembrane feed pump, second feed pump 84, which is then delivered to asecond membrane filter, second pass filter process 90, through a secondpurification feed pump line, line 87; the remaining water proceedsthrough a first membrane usable line, line 72.

Second pass filter process 90 has a membrane 112 which separatesrelatively salty water 111 from relatively less salty water 113.Controller 10 adjusts the pressure of the water delivered to membranefilter by input from pressure sensor 88, which is located in line 87,and controlling pump 84. The controller 10 may also adjust the waterpressure delivered by the pump 84 by measuring the differential pressurein the second stage filter 90.

The relatively salty water leaves second pass filter process 90 througha second membrane unusable line, line 92, where it subsequently mergeswith line 78 and proceeds into a purification unusable output line, line96, and directed into discharge 14. Furthermore, the application of anenergy recovery turbine 82 located in line 78 is an additional mechanismutilized to recover energy from the system.

Highly purified water, the relatively less salty water, leaves secondpass filter process 90 through a second membrane usable line, line 94,where it subsequently merges with line 72 producing water within adesired range of salinity, and proceeds into a purified line, line 68,and that water is directed into a storage tank, first storage tank 66.Controller 10 measures the salinity of the output water from salinitysensor 98 which is located in line 68. Water drawn from storage tank 66by a pump, pump 64, by way of a first storage output line, line 59,proceeds through a treated line, line 60, where it undergoes watertreatment 62 (typically lime stabilization and chlorination); then thewater is delivered to storage tank 67 for later use as the finishedpurified water.

As the temperature of the source water rises and/or the feed watersalinity decreases, the differential membrane pressure (e.g., totalpower cost) decreases and product water salinity increases. When productwater salinity exceeds the target water quality level, in a preferredembodiment the flow rate of the low-temperature pump (e.g., the ambientwater pump) is increased automatically (through the VFDs of thelow-temperature pump based on on-line temperature and salinity sensorreadings) to bring the product water salinity at the target level.Thereby, by automatic adjustment of the low-temperature feed pump flowrate, desalinated water of target quality is always produced at minimumpressure and power costs eliminating the negative effect of thefluctuations of feed water temperature and salinity on the overalldesalination costs.

Although this embodiment suggests seawater at the supply source for feedwater, any salty or brackish water supply may serve as the source.Furthermore, although a reverse osmosis desalination systems is shown,other desalination systems might also be utilized. Additionally,different variations of a desalination system would satisfy therequirements of the present invention, including varying the number andtype of membranes utilized, the use of nano-filtration, varying waterpressure, and the mixed use of natural and reverse osmotic approaches.Additionally, the disposition of sensors, mixers, blenders, pumps, andother elements in the above description and accompanying FIGURE may bemodified and retain the spirit of the inventions.

The above description and drawings are only illustrative of preferredembodiments of the present inventions, and are not intended to limit thepresent inventions thereto. Any subject matter or modification thereofwhich comes within the spirit and scope of the following claims is to beconsidered part of the present inventions.

1-17. (canceled)
 18. A method of desalinating water comprising the stepsof: receiving a first portion of salty water from a salty water intake,the salty water intake being configured for providing input salty waterto an electricity generating power plant; and desalinating the firstportion of the salty water at a desalination plant.
 19. The method ofclaim 18, further comprising: receiving a second portion of salty waterthat has been heated in the electricity generating power plant to atemperature higher than a temperature of the first portion of the saltywater, the second portion including at least some of the input saltywater; and heating the first portion of the salty water using the secondportion of the salty water.
 20. The method of claim 19, wherein theheating step includes mixing the first portion of the salty water withthe second portion of the salty water; and wherein the method furthercomprises desalinating the mixed first portion of the salty water andsecond portion of the salty water.
 21. The method of claim 19, whereinthe second portion of the salty water has been heated in a coolingsystem of the electricity generating power plant.
 22. The method ofclaim 18, further comprising: separating the first portion of the saltywater into usable water and unusable water in the desalination plant;and outputting unusable water from the desalination plant through adischarge.
 23. The method of claim 22, wherein the discharge isconfigured for discharging output water from the power plant.
 24. Themethod of claim 23, further comprising: mixing the output water from thepower plant with the unusable water from the desalination plant; andoutputting the mixed water through the discharge.
 25. The method ofclaim 18, wherein the salty water intake is located in a source ofseawater and wherein the salty water is seawater.
 26. The method ofclaim 18, wherein the salty water intake is located in a source ofbrackish water and wherein the salty water is brackish water.
 27. Amethod of desalinating water comprising the steps of: inputting a firstportion of salty water into a desalination plant; desalinating the firstportion of salty water in the desalination plant to produce a permeateand a condensate; and outputting at least part of the condensate througha discharge, wherein the discharge is configured for discharging outputwater from an electricity generating power plant.
 28. The method ofclaim 27, further comprising: receiving a second portion of salty waterthat has been heated in the electricity generating power plant to atemperature higher than a temperature of the first portion of saltywater; and heating the first portion of salty water using the secondportion of salty water.
 29. The method of claim 27, wherein the heatingstep includes mixing the first portion of salty water with the secondportion of salty water; and wherein the method further comprisesdesalinating the mixed first portion of salty water and second portionof salty water.
 30. The method of claim 27, further comprising:inputting the first portion of salty water and an input salty water tothe electricity generating power plant through the same intake.
 31. Themethod of claim 27, further comprising: mixing the at least part of thecondensate and at least part of the output water from the electricitygenerating power plant before discharging the mixed water through thedischarge.
 32. A desalination plant, comprising: a desalination processconfigured to receive salty water from a salty water intake, wherein thesalty water intake is configured to provide a first portion of saltywater to the desalination plant and to provide an input salty water toan electricity generating power plant.
 33. The desalination plant ofclaim 32, wherein the desalination plant is connected to a dischargearranged to output at least part of the first portion of salty waterfrom the desalination process and output water from the electricitygenerating power plant.
 34. The desalination plant of claim 32, furthercomprising: a blender for blending the first portion of salty water witha second portion of salty water that has been heated by the electricitygenerating power plant.
 35. The desalination plant of claim 32, whereinthe input comprises a filter for filtering the first portion of saltywater and the input salty water.
 36. The desalination plant of claim 32,wherein the intake is located in a source of seawater and wherein thesalty water is seawater.
 37. The desalination plant of claim 32, whereinthe intake is located in a source of brackish water and wherein thesalty water is brackish water.